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Consider four to five flights maximum if you don’t have
a way to check the batteries, and be sure to turn your
equipment off between flights.
If for any reason you think you might have a problem,
ask another flier for assistance. Many experienced
fliers have battery checking and field-charging
equipment on hand and would be happy to help.
If you are thinking about purchasing extra equipment, I would
recommend buying a digital voltmeter with an internal load
specifically designed for RC use (I use a Hobbico. It cost about
$25).
Before digital became popular, there were analog Ni-Cd
checkers. Expanded Scale Voltmeters (Hobbico still makes
these at around $12) provide a scale expansion that allows
more accurate reading around the voltages of battery packs
(the 4.8 and 9.6 volts).
Why expanded scale or digital? NiCads (and also Nickel-Metal
Hydrides - NiMH) are known to have a relatively flat voltage-
discharge curve. In other words, as they progress from fully
charged to fully discharged, the voltage decreases very little.
For this reason it is difficult to measure the battery’s charge
state without an accurate meter where you can see the small
differences between the two. You also must have some
knowledge of what the battery usually measures to see the
change.
The load feature puts a brief 75 to 200 mA load on the battery.
Always measure battery voltage under some load in order to
see how voltage holds under typical discharge load.
The best defense against the battery failure, and/or the
inadvertent “fly until discharged” crash, is frequent checking
under load with an accurate voltmeter.
You will hear fliers talk of cyclers that test and exercise
batteries. These are good, but not necessary.
A cycler will discharge a battery and count how many
milliamperes per unit time (milliamperehours) the battery will
supply while maintaining voltage above a certain voltage
(typically 1.1 volts per cell).
I use a cycler sometimes, but it basically is detecting early loss
of voltage during discharge. Occasionally checking batteries
under load with a simple voltmeter essentially accomplishes the
same thing.
Know your battery’s voltage history. Know that they are fully
charged for the start of your session.
Check the voltage before your first flight, maybe after the third,
and any other subsequent flights.
You will be doing the most you can to avoid the third most
common cause of pilot error—the error of not paying proper
attention to your equipment.
from the Indianapolis RC South club, Indianapolis IN
Our transmitters often use eight of the same cells in a series
resulting in a nominal 9.6 volts (1.2v per cell x 8). Transmitters
usually draw a constant current level of approximately 150 to
250 mA while transmitting.
Flight packs typically draw 30-60 mA when idle, but when flying
the servo motors are in constant use drawing higher currents.
Two standard servos can draw peaks of more than 400 mA. If a
flight surface is a bit stiff, servo current draw can increase
considerably.
The wall chargers supplied with typical radios do a fine job.
They charge at a relatively constant current of 50-70 milliamps.
This is one-tenth of the battery capacity specification.
These chargers are known as one tenth-C, or slow chargers.
This is the most reliable and simple arrangement, because
almost all Ni-Cds can tolerate considerable overcharge (days or
even weeks) if the charge current is one tenth-C or less.
Higher charging schemes need charge-end detection and
automatic shutdown in order to prevent overcharge damage.
Sounds complex? It’s not so bad. There is much you can do to
enhance your reliability without spending money on extra
equipment. For starters, here is a list of good practice items:
Protect the battery pack from excessive vibration by
wrapping a layer of foam around it.
Make sure you have a good charge before flying—a
full 10-12 hours. If you know your batteries are low
give them a full 18-24 hours.
Avoid using a wall socket controlled by a switch. It
might get turned off. Confirm charging by making sure
the LEDs are lit.
Batteries self-discharge slowly over time. Batteries can
differ in this area, and older batteries can lose charge
more quickly. If you charged your batteries
immediately after last week’s flights, and you plan on
flying tomorrow—charge them again. You want them
at their best.
Keep connections clean and in good shape.
Typical transmitters have a battery meter, display, or
LEDs to help monitor the transmitter. Learn how yours
reacts when batteries are new. What does a normal
full charge look like? How about after a half hour of
use? If it begins to behave differently, have it checked
out.
Batteries that are in their third flying season deserve
more attention. With fourth and fifth season batteries
you can almost expect a failure. Typically it will be a
single-cell failing, but do not trust the other cells unless
the pack is new. Individual cells can be replaced, but
it’s typically not worthwhile. A four-year-old pack with
one bad cell replaced will probably give trouble again
very soon.
With a full charge, how do the servos act? Are they
responsive and quick? If you ever develop a sluggish
servo get it checked out.